This invention relates to methods and means for producing a powder with non-linear voltage-current properties, that can be useful, for example, in eliminating corona in high voltage generators or other electrical machines having conductor extensions disposed in air or other gaseous medium. The dangers of corona, in high voltage machines are well known. For example, high voltage stator coils generally require the use of electrical stress grading systems along the exterior end portions of the coil for corona suppression. Without stress grading, the electric field along the surface of the coil can become sufficiently large, so that the air layer adjacent to the coil can break down, producing local corona and/or flashover from the high voltage leads to ground.
Several methods of preventing corona discharge and short-circuiting have been used. Berg et al., in U.S. Pat. No. 3,210,461, disclosed coated, insulated, exterior stator coil portions next to the grounded stator laminations, with a 10 mil thick coating of a semiconducting material. This material consisted of 1 part varnish binder and about 6 parts of finely divided non-linear silicon carbide powder, containing up to 4 wt.% of finely divided carbon. The resistivity of these silicon carbide coatings was non-linear, i.e., the resistivity varied with the voltage.
This method provided a useful stress grading for medium voltage machines operating at about a 25 KV voltage class. However, silicon carbide is abrasive, can be oxidized, and its manufacturing process does not yield easily reproducible results. In addition, its non-ohmic exponent of between 3 and 7 could be improved upon. What is needed is a powder having non-linear electrical properties with a non-ohmic exponent greater than 7.